1. Field of the Invention
The present invention relates to a drive circuit for turning a semiconductor device with a control electrode ON and OFF.
2. Description of the Related Art
Generally, a semiconductor device, upon being switched with a high frequency wave, fails to operate at high speeds due to the presence of a stray capacitance or inductance. This phenomenon will be set forth below by taking an IGBT, that is, a current switching type power device, by way of example.
FIG. 1 shows a conventional IGBT 7 and its associated gate drive circuit. Reference numeral 1 shows a turn-ON power source; 11, a turn-OFF power source; 2 and 12 current-limiting resistors; and 4 and 14, switches for power source switching. A stray capacitance 7a and leakage resistance 7b are created across a gate-to-source circuit of IGBT 7. Stray inductances 3, 13 and 5 are present in a gate drive circuit.
FIG. 2 shows an operation timing of the gate drive circuit of such a type as set out above. A current increase rate is restricted by an averse effect of the stray inductances 3, 13 and 5 when charging and discharging occur at the stray capacitance 7a across the gate of the IGBT 7. In FIG. 2, a broken line shows, for example, a case of a higher current increase rate (not a conventional case) and a solid line a case of a lower current increase rate (a conventional case). Let it be assumed that an ON control signal is supplied to IGBT 7 at time t0. In this case, a time until a start of the turning ON of IGBT 7 as indicated by a time t1' - t0 and a time from the start of the turning ON of IGBT 7 to an ON state as indicated by a time t2' - t1', both for a lower current increase rate, is greater than a time until a start of the turning ON of IGBT 7 as indicated by a time t1 - t0 and a time from the start of the turning ON of IGBT 7 to the ON state as indicated by a time t2 - t1, both for a higher current increase rate, respectively.
From this it will be evident that the respective times t1 - t0 and t1' - t0 as involved from the application of an ON control signal to the turning ON of the IGBT 7 are waste times. When a drive frequency of IGBT 7 goes high, such waste times are lengthened for a drive period, preventing a high frequency drive. The switching times t2 - t1 and t2' - t1' from the ON to the OFF state of IGBT 7 bear a close relation to an electric power loss of IGBT 7. An increase in length of the switching time leads to an increase in electric power loss of IGBT 7, failing to perform a high-frequency drive. In order to drive IGBT 7 in a high-frequency drive mode, it is necessary to supply an ON control signal to IGBT 7 at a higher current increase rate.
The current increase rate of the ON control signal is determined by the voltage E of the power source 1 and sum L of the circuit inductances 3 and 5 and defined as E/L. The value L of the stray inductance is determined by the inductance of component parts and connection wires of an assembled circuit and is more than several hundreds of n in terms of their physical conditions. Further, the voltage E of the power source 1 which is used for the ON control signal is of several tens of bolts because it cannot be made higher than the gate/source breakdown voltage of IGBT 7. Thus the current increase rate is restricted to less than several hundreds of A/.mu.sec. Here it is to be noted that the gate/source breakdown voltage of IGBT 7 means not a breakdown limitation level but a limitation voltage level allowable without degrading a reliability.
Although IGBT 7 has thus far been explained in connection with its turn-ON operation, the same thing can also be true in the turn-OFF operation of IGBT 7 provided that the polarities of the voltage and current are reversed in that case. That is, when an OFF control signal is applied at time t3, a time until a start of a turning ON of IGBT 7 as indicated by a time t4-t3 and a time from the start of the turning ON of IGBT 7 to the turn OFF of IGBT 7 as indicated by a time t5-54, both for a higher current increase rate, are smaller than a time until a start of a turning OFF of IGBT 7 as indicated by a time t4'-t3 and a time from the start of the turning OFF of IGBT 7 to the turning OFF as indicated by a time t5'-t4', both for a lower current increase rate. In order to drive IGBT 7 in a high-frequency drive mode, it is necessary to have a higher current increase rate in the same way as that in which IGBT 7 is turned ON. However it is not possible to obtain an adequately great current increase rate because the power source voltage is restricted by withstand voltage of IGBT 7 and the stray inductance of the circuit is present.
After IGBT 7 has reached a normal ON or a normal OFF state, it is only necessary to supply very small currents i.sub.lip, i.sub.lin as shown in FIG. 2 to the leakage resistance 7b connected across the gate and source of IGBT 7. In this respect, IGBT 7 is called a voltage-controlled type.
As set out above, in order to drive a control electrode-equipped semiconductor device at high speeds in ON/OFF drive fashion, a higher current increase rate is required for gate drive. However, problems arise from the fact that, for example, a power source voltage cannot be made higher due to a restriction imparted thereto by the withstand voltage of the device and that no adequately high current increase rate is obtained because the stray inductance of a drive circuit cannot be disregarded.